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BSC 219 Translation 10/22/12 15.1 Many Genes Encode Proteins The One Gene One Enzyme Hypothesis: Genes function by encoding enzymes, and each gene encodes a separate enzyme. More specific: one gene one polypeptide hypothesis Hypothesis that each gene encoded a single, unique polypeptide Codon: a triplet RNA code that corresponds to an amino acid in a protein Degenerate code: Amino acid may be specified by more than one codon. Synonymous codons: codons that specify the same amino acid Isoaccepting tRNAs: different tRNAs that accept the same amino acid but have different anticodons Wobble hypothesis-the third position of a codon has some variability. Sense codons: encoding amino acid Initiation codon: AUG Termination codon: UAA, UAG, UGA 15.1 Many Genes Encode Proteins The One Gene One Enzyme Hypothesis: Genes function by encoding enzymes, and each gene encodes a separate enzyme. More specific: one gene one polypeptide hypothesis 15.2 The Genetic Code Determines How the Nucleotide Sequence Specifies the Amino Acid Sequence of a Protein Reading frame: three ways in which the sequence can be read in groups of three. Each different way of reading encodes a different amino acid sequence. Nonoverlapping: A single nucleotide may not be included in more than one codon. The universality of the code: near universal, with some exceptions Aminoacyl-tRNA syntheses and tRNA charging The specificity between an amino acid and its tRNA is determined by each individual aminoacyl-tRNA synthesis. There are exactly 20 different aminoacylt-tRNA syntheses in a cell. Initiation factors IF-3, initiator tRNA with N-formylmethionine attached to form fmet-tRNA Energy molecule: GTP The Shine–Dalgarno consensus sequence in bacterial cells is recognized by the small unit of ribosome. AG rich sequence approximately 6-8 nucleotides upstream of initiation AUG The Kozak sequence in eukaryotic cells facilitates the identification of the start codon. ACCAUGG Exit site E-where depleted tRNA leaves ribosome Peptidyl site P –growing polypeptide attached to tRNA Aminoacyl site A-tRNA carrying next appropriate amino acid Elongation factors: Tu, Ts, and G Termination codons: UAA, UAG, and UGA Release factors-proteins that bind each of the termination codons and cause ribosome dissociation. The three-dimensional structure of the ribosome Polyribosomes: An mRNA with several ribosomes attached Messenger RNA surveillance: Detect and deal with errors in mRNA Nonsense – mediated mRNA decay: eliminating mRNA containing premature termination codons The posttranslational modifications of proteins Glycosylation in ER and Golgi Cleavage by specific enzymes Specific folding by chaperones Ribosomes that stall at the end of an mRNA molecule without hitting a termination codon can be released by a special mechanism. Transfer-messenger RNA binds to A site of ribosome and acts as template. Alanine is initially added, then 10 amino acids encoded by tmRNA These serve as tag for rapid degradation of protein Termination codon is contained on tmRNA and termination takes place